In an optical communication technology or an optical measurement technology, an optical modulator in which an optical waveguide and a control electrode are incorporated into a substrate having an electro-optic effect is frequently used. In such an optical modulator, a Mach-Zehnder type optical waveguide is formed in the optical waveguide, and in the control electrode which controls light waves propagating through the optical waveguide, in a case where, for example, LiNbO3 is used for the substrate, a signal electrode and a ground electrode is formed in a thickness of several tens of μm. In the optical modulator in which LiNbO3 is used for the substrate, it is known that an operating point shift, a so-called temperature drift, which is caused by a temperature change, occurs.
In an optical modulator using a Mach-Zehnder type optical waveguide, for example, techniques of improving a temperature drift characteristic by taking the balance of stress affecting the optical waveguide due to a control electrode by securing the symmetry of cross section structure of the control electrode with respect to the optical waveguide are disclosed in Patent Literature Nos. 1 to 4 as well. Patent Literature No. 1 discloses a technique in which a signal electrode and a ground electrode facing it are formed substantially bilateral-symmetrically against the center between two of branching waveguides in a modulation region of a Mach-Zehnder type optical waveguide. Further, in order for the structure of a part of the ground electrode to be the same as the signal electrode, the thickness of the ground electrode is formed to be thinned at a specific region of the ground electrode.
Patent Literature No. 2 discloses a technique in which with respect to the thinned electrode of the specific region of the ground electrode of Patent Literature No. 1, a portion in which a conductor is lacked partially is formed, and thus the influence of stress of an outside part (the ground electrode which is present in a region away from an optical waveguide) of the ground electrode on the optical waveguide is suppressed. Further, Patent Literature No. 3 discloses a technique in which a part which is lack of a conductor partially is formed in each of two ground electrodes which put a signal electrode therebetween, so as to be symmetrical with respect to a center line of the signal electrode (a central conductor).
Further, Patent Literature No. 4 discloses a technique in which in a case where a plurality of Mach-Zehnder type optical waveguides are disposed in parallel, the structures of a signal electrode and a ground electrode are formed so as to be symmetrical with respect to not only the center between two branching optical waveguides configuring each Mach-Zehnder type optical waveguide but also the center between the Mach-Zehnder type optical waveguides adjacent to each other.
In a recent optical modulator, in order to meet market needs of larger-capacity and higher-speed in optical communication, a multi-level modulation format becomes to be used, and high-integration such as disposing a large number of Mach-Zehnder type optical waveguides in parallel is on-going, and an optical transmitter part which includes an optical modulator also requires a reduction in power dissipation or a down-sizing. Further, also in a transponder implemented a modulator, larger-capacity and higher-speed, a reduction in power dissipation, and a down-sizing are on-going, and in a drive circuit of an optical modulator, suppression of degradation of characteristics is required by simplification. For example, in the case of an optical modulator having a structure in which a RF modulation electrode part which superimposes signal components on each other and a DC electrode part which adjusts an operating point are combined, in order to protect a driver amplifier for driving the optical modulator from DC voltage, a DC block capacitor having a large withstand voltage is required in a front stage of the modulator. For this reason, an AC/DC separating electrode structure is desired in which the RF modulation electrode part and the DC electrode part are independently disposed and the DC block capacitor which is disposed in the front stage of the modulator is not required. In such a modulator, a control electrode or a wiring structure thereof is more complicated, and therefore, it has been getting difficult to be satisfied with symmetry of a cross section structure of an electrode against a waveguide in each Mach-Zehnder type optical waveguide. Further, stress imbalance due to an asymmetric structure causes different refractive-index changes in optical waveguides of arms of a Mach-Zehnder in accordance with a temperature change, and thus a phase difference is generated between the arms of the Mach-Zehnder, and as a result, an operating point shift, a so-called temperature drift phenomenon, is generated, whereby it becomes problematic.
In particular, in the AC/DC separating electrode structure, in order to suppress an increase in the size of the optical modulator itself, a longer DC electrode cannot be secured, and thus the Vπ voltage of a Mach-Zehnder structure becomes higher. Further, the temperature drift phenomenon occurring in one Mach-Zehnder structure is a sum of the temperature drift phenomena by the DC electrode and the RF modulation electrode superimpose, and therefore, the value of the DC voltage which is required to compensate for the operating point shift becomes larger, compared to a structure in which the RF modulation electrode and the DC electrode are combined. Furthermore, the operating point shift amount due to a DC drift phenomenon is proportional to the magnitude of the DC voltage which is applied, and therefore, in the AC/DC separating electrode structure, as described above, a large DC voltage is required for bias point compensation, and therefore, a large DC drift is induced, so that it makes difficult to assure a long term operation of an optical modulator. Due to these, it is indispensable to further suppress the operating point shift due to the temperature drift.
The inventors of the present invention have performed intensive studies with respect to a cause of the operating point shift based on the temperature drift phenomenon in a highly-integrated optical modulator. As a result, the inventors have found that if integration is required, as shown in FIG. 1, it is necessary to dispose a number of signal electrodes (S1 and S2, includes a RF modulation electrode or a DC electrode) on a substrate in which a plurality of Mach-Zehnder type optical waveguides (branching waveguides L1 to L4) are formed, and leading-out of the signal electrode is complicated, and due to a requirement of a down-sizing of an optical modulator, lead-out wiring of the signal electrodes has to be arranged close to the optical waveguides within a limited space, as shown in portions surrounded by frames a and b, for example, and stress acting on the optical waveguide becomes different between the optical waveguides, and therefore, it is one of the causes of the operating point shift based on the temperature drift phenomenon.
In particular, as the reason why a detour portion of the lead-out wiring of the signal electrode increases and the wiring is complicated, not only the integration of the Mach-Zehnder type optical waveguides (a nested optical waveguide or the like) but also making the electrical length of signal wiring from an electrical input pad part of each signal electrode to an interaction part of the Mach-Zehnder type optical waveguide be the same between the respective signal electrodes in order to match a so-called skew, or concentrating the electrical input pad parts on one of side faces of the substrate in the optical modulator, or the like can be given. Further, the width (in a direction perpendicular to a light propagation direction) of a chip of an optical modulator is limited, and therefore, the lead-out wirings being arranged to be integrated within a narrow space is also one of the reasons.
FIG. 1 shows a plan view of the vicinity of a RF modulation signal input part of an optical modulator, and FIG. 2 is a cross-sectional view taken along a dot-and-dash line A in FIG. 1. As shown in FIG. 2, in a case where the widths of ground electrodes (a width w2 of a ground electrode G2-1 and a width w1 of a ground electrode G3-1) on both sides of a signal electrode S2-1 acting on the optical waveguides L1 and L2 with the signal electrode S2-1 as the center (a dot-and-dash line) are different from each other (w1≠w2), the electrode dispositions with respect to the optical waveguides L1 and L2 become different from each other. In a case where a temperature is changed, a change of strain caused by stress to the substrate (LiNbO3 or the like, also referred to as an LN substrate) is generated due to the expansion and contraction of a metal film configuring the electrode. However, in the case of FIG. 2, a change of strain with respect to each of the optical waveguides L1 and L2 becomes different due to the asymmetry of the electrode disposition, and the difference of strain induces a phase difference between the optical waveguides (L1 and L2). That is, a (temperature drift) phenomenon in which a bias point of the optical modulator changes occurs. In particular, if the electrode thickness is 10 μm or more, a strain due to stress which occurs becomes larger, and therefore, the influence becomes remarkable.
As a method of solving such problems, for example, a method of forming a recess portion (a thin portion of an electrode) at a position of the width w2 from the signal electrode S2-1 side on the ground electrode G3-1, as in Patent Literature No. 1, or a method of making the width of the ground electrode G3-1 be the same as the width w2 of the ground electrode G2-1, as in Patent Literature No. 4, is conceivable. However, in the former method, even if a structure of the ground electrode is adjusted, lead-out wiring S2-2 is present in the vicinity thereof, and therefore, a configuration of suppressing a strain due to stress corresponding to the lead-out wiring is further required, and it becomes a reason to make the electrode structure further complicated. Further, in the latter method, in a case where the width of the ground electrode G3-1 is set to be narrow so as to be equal to w2, the lead-out wiring (a portion a of FIG. 1) comes closer to the optical waveguide, and thus the influence of a strain due to stress which is applied by the lead-out wiring becomes larger.
Moreover, the lead-out wiring which is disposed close to the optical waveguide is present in not only the vicinity of a region of an interaction part between the signal electrode and the optical waveguide (indicated by an arrow R1 of FIG. 1) but also a region except for the region of the interaction part, as shown in the frame b, and it is indispensable to take into account the influence of the lead-out wiring in the entire region in which the branching waveguides of the Mach-Zehnder type optical waveguide are formed (indicated by an arrow R2 of FIG. 1).